NEW YORK ( TheStreet) -- Looking for a new landing spot in your career? Make sure your cover letter is in good shape, because you'll need it. In fact, 91% of hiring managers consider cover letters "valuable" when vetting potential employees, according to a study last year from OfficeTeam. Another 79% say they expect cover letters with resumes from people applying for a job, even though many resumes are now sent online. "Although the job application process has increasingly moved online, the importance of a cover letter shouldn't be underestimated," says Robert Hosking, executive director of OfficeTeam. "It often is the first opportunity to make a positive impression on hiring managers. It's also a chance to provide context for your resume, expand on key accomplishments and explain reasons for employment gaps or career changes." Problems can start right up top on a cover letter, and often roll right through the rest of the document. Here are five of the biggest mistakes job applicants make when creating and submitting one: resume and cover letter may well be the first eliminated. Another tip: Address the hiring manager by name (and not by "To whom it may concern"). If you don't know the manager's name, call the firm and ask, or check the company's website. Going on too long. Legendary author Ernest Hemingway once said brevity was key to his literary success, and he was rumored to have won a bet that he could write a story in six words (his entry: "For sale: Baby shoes. Never worn.") Cover letter writers could learn from his example: Keep the letter to a single page. Hiring managers are busy and won't read a lengthy monologue on your life and career. Stick to the facts, and your accomplishments, in an organized fashion. Mirroring your resume. Too often, human resources managers, as the gatekeepers of a company's hiring machine, will toss a cover letter because they see it as just a reproduction of your resume. There is a difference between the documents. A resume is a timeline of your career and accomplishments and a good cover letter amplifies that resume, giving you some room to elaborate on your accomplishments, explain gaps in employment and tailor your accomplishments to a specific employer's needs.